BLM Technical Note 380
'^- — *•*
As the Nation's principal conservation agency, the Department of
the Interior has basic responsibilities for water, fish, wildlife, mineral,
land, park, and recreational resources. Indian and Territorial affairs are
other major concerns of this department of natural resources.
The Department works to assure the wisest choice in managing
all our resources so that each shall make its full contribution to a better
United States now and in the future.
BLM Technical Note 380
INTERPRETATION OF AERIAL PHOTOGRAPHS
Wallace A. Crisco
BLM Service Center
Available from: Printed Materials Distribution Section
BLM Service Center (D-558B)
Denver, CO 80225-0047
National Technical Information Service
Springfield, VA 22161
U.S. Department of the Interior
Bureau of Land Management <l^
This publication is designed to provide the necessary content for a basic
aerial photo interpretation course. The order in which subjects are addressed
may need to be varied. The depth of instruction about various portions may
need modification depending on the experience or training background of the
audience. It is hoped that the utilization of this material in conjunction
with site specific information (field site photo's and other data) will
provide the basic components (off the shelf) for many Photo Interpretation
TABLE OF CONTENTS
1. Introduction L
2. Elements of Aerial Photography
o Types of Photography 2-3
o Fi 1ms 4
o Filters 4
o Photographs 5
o Photography Acquisition 6-7
o Titling and Indexing 8-9
o Displacements and Distortions 10-13
o Scale Considerations 14-17
3. Photo Interpretation and Usage
o Scale Determinations 18-19
o Acreage Determination 20-21
o Image Characteristics 22-31
o Techniques and Procedures 32
o Photo Preparation 33-35
o Vegetation Overlay 37
o Drainage Overlay 39
INTERPRETATION OF AERIAL PHOTOGRAPHS
This publication deals with aerial photographs and how they can be used in
the various phases of land management within the Bureau of Land Management.
It is intended to furnish sufficient guidelines to encourage the use of aerial
photos. Emphasis has been placed on basic data in this publication for its
general application to Bureau activities where aerial photos can be
The aerial photograph offers the easiest, most complete way to examine an
area in it's entirety. It's unique portrayal of all visible detail makes it
possible to analyze, measure, quantify and interpret more information than can
be obtained from any other single source. It has revolutionized topographic
mapping and brought new search techniques to such diverse fields as geology
An aerial photograph is simply a photograph of the surface of the earth.
It may approximate the normal view from an elevation but more likely it will
have the unfamiliar view of the scene directly below the airplane. The Bureau
can make classifications of land with a high degree of accuracy by skilled
photo interpreters. Soils, building materials, and timber stands are being
accurately determined in the same manner. The increased workload and
decreased workforce necessitates advanced planning and programming to utilize
photo interpretation to collect and analyze reliable data for managing the
Even the fact that a photograph grows old is of value. The record made by
a photograph is so inclusive at tne time it is taken that it becomes a
valuable historical document and will serve as an excellent reference for
measuring our progress and accomplishments on the land itself.
Recognition of the value of the aerial photograph comes from use and
understanding of the photograph's limitations as well as its capabilities. If
fully utilized, aerial photographs are a tool to make our work easier and
enables us to do it better. This publication is aimed at presentation of
fundamentals in the use of aerial photography.
i<~ — "
Most commonly used
Low oblique photography
(Horizon is not included)
Less commonly used
High oblique photography
r \ ,. , 5 /-' RELATION OF
'&WM*PW£ CAMERA TO
r\^'W^r^% THE GROUND.
^*-^ ^i&SWt&fr. ^
(Horizon Is included)
Figure I. Types of aerial photography
ELEMENTS OF AERIAL PHOTOGRAPHY
Types of photography
Three common types of aerial photography are: Vertical, Low oblique, and
High oblique (Figure 1).
The vertical photograph is taken from a viewpoint directly below the
airplane. This gives a map-like photograph of the earth's surface, however,
it is not a map. The map-like qualities are emphasized only where the terrain
is flat and level. A grid in that case will be relatively square and suitable
planimetric maps can be prepared directly by overlays because corrections are
easy. Distortions and displacements will require some corrections.
This is the most commonly used photograph. All parts of the terrain are
visible, scale is fairly uniform throughout, except in mountainous terrain,
and complex methods are not needed to make simple maps. A three-dimensional
effect of terrain can be easily seen with the aid of a stereoscope.
Oblique photographs have one factor in common; the camera is at a
predetermined angle to the ground. Division into low and high oblique
photographs refers only to the angle not to the altitude of the camera.
The low oblique photograph can be defined as an angled camera shot that
does not include the horizon. The angle provides a panoramic view of a larger
area than does the vertical photograph. The photographs may be "restituted"
by rephotography in the darkroom and made into verticals if their angles were
low. Higher angles require special mapping techniques. Obliques may be used
to bring maps up-to-date in small areas but the methods are tedious for larger
Principal use of this type of photograph is usually limited to the
large-scale single photograph taken at low altitude, for public education,
reports, demonstrations, and similar needs.
The high oblique photograph includes the horizon. The perspective is
natural or the way we normally see things. Reference to objects in the
photograph is readily understandable because of its familiar angle of view.
It is therefore, very good for displays, presentation of entire watersheds or
other problems, especially with the public, where quick reference and
recognition are desirable. The limitations of high oblique photography come
in mountainous or hilly areas as slopes facing away from the camera are unseen
In summary , the Bureau confines most of its use to the vertical
photograph because it requires minimum equipment to record the information in
a usable form. This is related in turn to its relatively uniform scale, the
map-like presentation of information, and its general availability and
Obliques are more specialized. The specialization is advantageous in
that the perspective may approach a normal viewpoint, thus interpretation is
easy for the average person.
All types demand infinitely greater integration into Bureau programs as
they will perform services that cannot be achieved by other means.
The film used for most aerial photography is a dimensionally stable base
film (e.g., Estar Base). Project requirements normally dictate whether black
and white, color negative, color positive, or color infrared film should be
Black and white photography of metric quality is most commonly used for
photogrammetric operations such as precision topographic mapping and riparian
Natural color (positive or negative film) is widely used by geologists
and others to locate potential oil well drill sites as well as other energy
sources. It is used extensively for resource inventories such as Forestry,
Range, Wildlife, watershed, and others.
Strip mining operations can be accurately monitored and trespass verified
by proper utilization of this tool. There are many other applications for
this kind of imagery in various disciplines.
Color infrared (CIR) is extremely helpful in vegetative inventory work
because of the ease in differentiating between species. Consequently it
speeds up the interpretation process and increases accuracy of the interpreted
Another application of CIR is in inventorying water sources (such as
springs). When properly used the wet areas stand out clearly from surrounding
vegetation. Most of the applications listed under natural color can also be
accomplished with CIR.
Filters are used for many purposes with both black-and-white and color
films. The filters vary the contrast and tonal rendering of the subject in a
photography, either to correct to the normal visual appearance or to
accentuate special features. The photographic effect obtained with a
particular filter depends on four main factors: (1) the filter's spectral
absorption characteristics, (2) the spectral sensitivity of the sensitized
material, (3) the color of the subject to be photographed, and (4) the
spectral quality of the illuminant.
Aerial views, when photographed without a filter, often appear veiled by
atmospheric haze. This condition is caused by scattering of ultraviolet and
blue radiation by water vapor and dust particles in the atmosphere. The high
sensitivity of aerial films to these radiations compounds the effect. Since
atmospheric haze scatters decreasing amounts of green and red light, and
virtually no infrared radiation, haze filters are primarily ultraviolet and/or
blue absorbers. Filters such as the KODAK WRATTEN Filters No. 2B and No. 2E
absorb ultraviolet to reduce haze effect without affecting the monochromatic
rendering of other colors.
Greater haze reduction results from the use of yellow or red filters
which absorb the shorter wavelengths. However, these filters also affect the
monochromatic rendering of colors. The greatest penetration of atmospheric
haze occurs when an infrared-sensitive material is used with a suitable filter
Most aerial films require some type of filter in the camera system to
eliminate the effects of atmospheric haze. Haze imparts an overall bluish
cast to color aerial photographs and lowers the contrast in aerial
black-and-white negatives. Filters that are appropriate to a particular film
have been indicated in the data section for that film. In addition to the
preceeding information fro"m Kodak publication M-29 , other technical data about
filters may be obtained from Kodak publications M-5 , and B-3 as well as other
There are many types of photographic materials available that produce
varying results, depending on the emphasis desired. Therefore, one should be
aware of this in order to acquire the kind of product needed for a specific
project. There are numerous kinds of paper that render different surface
characteristics such as: glossy; high lustre; lustre; semi-matt; and matt.
All of these have different degrees of smoothness or roughness that may
require special pens or pencils to write on them. Various contrasts are
available as well as different weights (thickness) of materials. The most
commonly used for color photos is a resin-coated water resistant base paper,
and probably a polycontrast paper would be the most common product used for
black and white photo's.
Also, there are stable base materials (similar to mylar) such as Kodak
Duratrans Display Film, and Cronapaque by Dupont. Duratrans yields a color
product and Cronapaque yields a black and white product that is scale stable.
Of course, these will cost more than the paper products.
A general rule is that the smoother the surface, the sharper the detail
but the more difficult it is to write on. If you use an overlay it shouldn't
Considerat ions for planning a photo mission: 1) Intended use of the
photography, 2) Who will fund the project, 3) Scope or size of project (how
many square miles or lineal miles), 4) Season (start and completion dates)
(What are you trying to emphasize), 5) Time of day to fly (especially for
cliff areas), 6) Camera focal-length (3-1/2, 6, 8-1/4, or 12 inch), 7) Scale
(depends on camera focal length and project needs), 8) Type of film (black. &
white, color, or CIR) , 9) Type and number of prints (1 set contacts, or ?),
10) Special requirements (film to wind in direction of flight, new film, etc.)
BLM standard or special aerial photography specifications should be used
to assure consistant quality imagery. Some of the specific requirements are
discussed and illustrated in the following pages.
Aerial photographs are taken on clear days when the sun is high enough to
avoid distratingly heavy ground shadows. The project is flown at a specified
flight height above mean terrain throughout the area. Each flight must
parallel those adjoining for blocked areas that require multiple flights.
Forward Overlap--usually 60% alon^
line of flight
Line of flight
Side overlap — usually 25% to 30% along
adjacent flight strips.
The ideal flight pattern is difficult to maintain. Sidewinds require
flight corrections, the camera may be at an angle to the flight, then forward
overlap and side overlap will grow inadequate with "crab." If the pilot fails
to correct for the wind he will "drift" off the line of flight. Both "crab"
and "drift" may require rephotography . See Figure 2 for an illustration of
crab and drift.
<■ •■■ s
-*■< *'i *\
A. Desired pattern of flight
r. . mi. ....
Figure 2. Index flight paths.
Titling and Indexing
Photographs are individually titled. Each photograph is numbered
consecutively as it was taken. Usually the date is included and the entire
series may be as shown in the following example.
Month /Day /Year
S 1 -
N 1 -
\ l "
These photograph numbers indicate "full" coverage,
Every exposure is necessary for stereoscopic work,
These photographs are "alternates." Cost is cut one-
half when prints are purchased. Does not give good
coverage for stereoscopic work — but will suffice for
simple planimetric maps.
Indexes are an effective way to catalogue photo coverage for future use.
An aerial photographic-index or spot (line) index may be used. These indexes
identify the aerial photograph with the terrain.
Aerial photo contractors usually will prepare a photo-index (photographic
index) at additional contract cost, which varies with CIR, color, or black and
white. Photo indexes are prepared by placing the photos in their relative
positions with individual identity accented; a large photograph is then made
of this group. If there are prominent features evident, the photo-index can
be used for location of desired photos. A measure of control is present;
townships can be superimposed on these photo-indexes with fair accuracy.
Unfortunately an index of this sort usually is difficult to read and use, see
Spot indexes or line indexes are relatively inexpensive, especially if BLM
provides the maps or matte mylars for their preparation to the contractor.
The spot index is plotted on a map base showing flight lines and exposure
stations. The land net on the map makes using this index very easy, see
* m.,m > » > i
U S DEPARTMENT OF AGRICULTURE
ROCKY MOUNTAIN REGION - R 2
INDEX OF AERIAL PHOTOGRAPHS
FOREST SERVICE QUAD NO 73
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Figure 2a. Typical photo-index.
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Figure 2b. Typical spot (line) index.
Displacements and Distortions
Displacement due to relief is common within the photograph. The effect of
elevation in photographic displacement is shown in Figure 3. The datum plane
is identified for reference. Point "A" on a hill properly should be located
on a true horizontal distance from the center of the photograph at "A-*- , " but
is displaced outward in the photograph to "A^." Point "B" is below the
datum plane. Its correct location on the datum plane is displaced toward the
photograph center, in this case to "B^." A matter for emphasis is that in
both situations the displacement of a point inward or outward is on a straight
line from the center of the photograph.
Displacement of a point then is caused by the effect of relief. No matter
to what degree points are displaced they move inward or outward on a line
radiating from the center. A compass bearing from the center to the object
point will not change with displacement. A hilltop, in the several
photographs surrounding it, is displaced outward on a line from each center.
Assume this center of each photograph is located correctly in its relative
ground position. Every line bearing on the hilltop from each center is on a
true bearing. The point where the lines intersect represents the true
horizontal location of the hilltop.
Intersecting rays from the centers of overlapping photographs are plotted
to locate points in true horizontal position and thereby eliminate
displacement due to relief. This is the basis of the "radial-line" and
"slotted-templet" method of map preparation. We can adapt this technique to
locate single objects, such as a new radar site on a hill.
Points above datum plane are displaced outward from the center.
Points below the datura plane are displaced toward the center.
Displacement is radial relative to the center of the photograph.
A to A = Distance of displacement away from center of photograph
B to B = Distance of displacement towards center of photograph
A and B = Corrected positions
A and B = Displaced positions on photograph.
Figure 3. Displacements and distortions.
Figure 4. Displacement due to relief.
Figure 4 illustrates a square section with rugged terrain. The centers of
the section and photograph coincide. The Sw and SE corners lie on the datum
plane, whereas, NW is above and NE is below it. NW will displace outward,
radially, from the center and NE will displace inward, radially, toward the
center because it is located in a deep canyon. The square section is
illustrated with solid lines; dotted lines show the distorted view of the
section as it is depicted in the photograph.
Distort ion in the photograph may be caused by several things, e.g., photolab
processing, film or paper instability, camera lens, tilt or tip of the camera
at the instant of exposure, etc.
Photolab processing accounts for some minor distortion in photographs.
The wet paper print may stretch in one direction and shrink, in another
dimension upon drying. Such errors are lessened by use of resin coated
paper. Single-weight paper is used in mosaics where stretching sometimes is
done purposely for a match of detail.
Distortion from the camera lens may be present. If the photographs are
enlarged, the enlarging may introduce further errors. This difficulty can be
minimized by confining your work, to the center of the photograph (or neat
model). Use more photographs if necessary. Modern "distortion-free" camera
lens have minimized this problem.
Tilt and tip causes the photo to have a distorted scale similar to that of
a low oblique photograph but is uncontrolled. Tilt is divided, sometimes,
into two parts--tips and tilt--for computing corrections. Tip in a photograph
is caused by the airplane climbing or diving at an angle to the ground. At
right angles to tip is "tilt" as the airplane banks in flight. Bureau
specifications state that tilt shall not average more than 2° in any flight
line nor more than 1° for the entire project. Relative tilt between any two
successive exposures exceeding 6° may be rejected.
Photographs contain invaluable information but also have errors
(displacements and distortions) which must be recognized and, if necessary,
corrected. You can correct many of these errors as you gain experience but
for now to minimize the problems:
From photolab processing; order resin coated or other stable base
From camera lens; use the center portion of photographs.
From tip and tilt; require photography of less than 3 percent error.
The scale of an aerial photograph is a vital piece of knowledge for your
work. It is the ratio of a distance on the photograph to its actual distance
on the ground. This ratio of photo distance: ground distance (PD:GD) is the
scale. It is expressed usually in three ways:
A. Descriptive scale
The descriptive scale is in common use. It is a statement of ratio in
familiar terms, such as "1/2 inch = 1 mile." It is awkward, however, because
two different units of distances are used, one for the ground and one for the
photograph. Inches on the photograph are related to miles on the ground.
B. Graphic scale
The graphic scale permits direct measurement on the photograph in a
convenient unit (miles, yards, feet). There is no calculation in its use;
hence it is rapid and easily used.
C. Representative fraction scale or RF
The representative fractions (RF) is the ratio of photograph distance
to ground distance with both distances expressed in the same unit of measure.
photo distance PD 1 GD
Scale = or — or PD:GD = corresponds to 1: —
ground distance GD GD/PD PD
This is the basic statement of scale for all map and photograph work. It is
best suited for calculations. Example of an RF is 1:63,360, which is the same
as stating "1 inch = 1 mile" (63,360 inches). Thus the RF is really the
familiar descriptive scale with the confusion of two units of measure removed.
The scale of an aerial photograph is determined by two factors at the moment
it is taken (Figure 5).
1. Height of airplane above the ground (not the altitude of the plane).
2. Focal length of the camera lens taking the photograph.
Problem: Scale is calculated by the ratio :H (focal length : Height
above-ground. Both in same unit of measure). Find the scale when airplane
altitude = 10,000', focal length of lens is 12", ground elevation is 3,000
Solution: Subtract 3,000' from altitude = 7,000', or H. f = 12 inches or 1
foot. Scale = f:H, or 1:7,000.
The scale of photographs is usually given as a representative fraction (RF)
and written on one line (1:5, 000). If a vertical photograph contains the
altitude and focal length of the camera, the scale is obtained as shown below.
Scale = RF=-j^- = f:H
Corresponds to I: -7-
f = Focal length
Note: f and H must be in the
same unit of measure.
Figure 5. Determination of photo scale from lens focal length and altitude
FOCAL LENGTH IN FEET
FLIGHT HEIGHT IN FEET " FH/FL
Scale on mesa top
.5' (FOCAL LENGTH) .
9,000 (ABOVE GROUND) " 18,000
I Mile = 3.5"
— Corresponds to I:—
Scale on valley floor
I Mile = 3.2"
Figure 6. Effect of elevation on scale
Scale varies with the height of the camera above the surface of the datum
plane or mean terrain elevation. It follows then that the scale varies within
each photograph with every elevation change. Figure 6 (Effect of elevation on
scale) shows this problem. However, the effect of terrain height on the photo
scale is not confined to such violent contrasts. A tall building will have a
different scale at its base from that of its roof.
You may be classifying a perfectly square section lying on a gently
sloping side hill with only a small grade (a rise of 300 feet in 1 mile). The
section will be deformed on a photograph. At a scale of 1:21,000, the low
side of the section will measure 3.0 inches; the higher side 3.1 inches--a 3%
PHOTOINTERPRETATION AND USAGE
A quad map is good for calculating the scale of a photograph. The same
points must be recognizable on both map and photograph. Each point should be
located as far distant from the other as possible and be within your project
The calculation of Figure 7 is typical for one pair of points. Here "A"
and "B" were recognizable on both map and aerial photograph. First determine
the actual ground distance between "A" and "B." The distance between them on
the map is 1.2 inches. Map scale is 1:50,000. Therefore, ground distance is
1.2" x 50,000 = 60,000 inches from "A" to "B." Now to determine the
photograph scale as the ratio of photo distance to ground distance (PD:GD).
PD is 2.5" on the photograph (by measurement) and GD is 60,000" (calculated
from the map). The ratio, therefore, is 2. 5" : 60,000. " Divide both parts of
the ratio by 2.5 which gives the answer: 2.5 f 2.5 = 1
60,000 v 2.5" = 24,000
which is the photo scale.
The scale of a photograph is easily determined in the field . A pair of
points are located on the photograph along roads or routes that can be
measured easily. Example: Assume the distance between two points on a road
is 1.7 miles and the distance measured on the photograph is 2.3 inches. Find
the photograph scale.
Converting miles to inches gives all measurements in a common unit (1.7 x
5280 x 12" = 107,712"). The scale PD:GD 2. 3 ' ; 107 , 712" = 1:46,831, or, in
round numbers, 1:47,000.
By comparison , the photograph will sometimes reveal square sections. A
quick measurement can give the inches per section on the photograph, perhaps
3" = 1 mile (3"=63 , 360") , which can be resolved by inspection into the nominal
scale of 1:21,000.
'hoto -scale unknown
Map -scale 1:50,000
Distance AB = 2.5" on photo
Distance AB = 1. 2" on map
Determine scale of photograph.
Scale s ratio of photo distance to
1.2" x 50,000 = 60,000" ground
distance between A and B .
Photo Scale -
2.5" + 2.5" =
60,000" -r 2.5" = 24,000
or 1 : 24,000
Figure 7. Determination of photo scale
Some typical uses of photo scale are for: locating legal subdivisions
(sectionizing) ; determining the size of objects on the ground such as dams,
buildings, recreation areas, etc.; and calculating approximate acreage
directly from the photographs.
Many field offices have access to digital planimeters that calculate the
acreage when a photo scale is entered. Another technique that can be used in
the field is the Dot Grid method. Examples for determining the value of a dot
and acreage calculation are as follows.
Dot value for Dot Grids example: Photo scale 1:24,000
Step one: Convert photo scale to feet per inch,
answer; 24,000 *■ 12 (inches per foot) = 2,000' or 1 inch of photo coverage
is equal to 2,000' on the ground.
Step two: Determine how many square feet are in 1 square inch at a scale of
1:24,000 or 1" = 2,000' .
answer; 2,000' x 2,000' = 4,000,000 square feet in 1 square inch of photo
coverage at 1:24,000 scale.
Step three: Determine how many acres are in 1 square inch at 1:24,000 scale.
answer; 4,000,000 sq. ft. *■ 43,560 (number of sq.ft. per acre) = 91.8
acres per square inch of photo coverage, at 1:24,000 scale.
Step four: Determine the value of each dot at a scale of 1:24,000
answer; 91.8(acres) *■ 100 (no. of dots per square inch) = .918 or .92
acres per dot.
Note: The reason for converting scale 1:24,000 to feet per inch is to get the
formula; feet times feet = square feet (4,000,000 in this example) and convert
that to acres. It also accommodates pocket calculator limitations.
Use of Dot Grid — randomly lay the dot grid over the area to be measured
(polygon, rectangle, square, etc.) and count each dot, (count the dots that
fall on the boundary line(s) as one half dot) and multiply the total by .92 to
determine the number of acres in an area at 1:24,000. This should be done two
or three times for each area and average the results.
Acreage Determinations Continued
The following examples show how acreage can be calculated on square,
rectangular, or triangular areas once the photo scale has been determined.
For both examples assume a photo scale of 1:24,000 or 1" on the photo is equal
to 2000 feet on the ground.
Example No. 1 - You have an area on the photo that is 1.1 inch wide and 3.4
Multiply 1.1" x 2,000' = 2,200' wide
Multiply 3.4" x 2,000* = 6,800' long
Width (2,200') x length (6,800') - 14,960,000 sq. ft. in the area
14,960,000 * 43,560 (no. sq. ft. in 1 acre) = 343.4 acres
(Note: This same formula works for a square area.)
Example No. 2 : You have a triangular area that is 1.1" high and 3.4" long as
Multiply 1.1" x 2,000' = 2,200' (height)
Multiply 3.4" x 2,000' = 6,800' (length)
2,200 x 6,800 = 7,480,000 sq. ft. in the triangle
7,480,000 4- 43,560 (no. sq. ft. in 1 acre) = 171.72 acres in triangle
Figure 8. Acreage determinations.
During interpretation of aerial photographs occasionally the terrain
features will "reverse" themselves; that is, the drainages appear as the
highest points. Turn the photographs around; this will force the features
into their proper relationship.
Direction is sometimes a very important aid to the interpreter. In
northern latitudes the shadows in the photograph fall toward the northwest if
taken in the morning and northeast for those taken in the afternoon. North
may be determined with greater certainty by comparing the photo with a map of
the area or orienting by compass. Resource photography flights are usually
flown in cardinal directions — east-west or north-south — with
photo-identification uniformly presented at one end of the photograph — the
west or north side. (SPECIAL FLIGHTS, e.g., stream photography or other
special projects may not be flown in cardinal directions, therefore, the
titled information may not indicate direction).
Some of the photo image characteristics that assist in the identification of
objects are illustrated in Figures 9 and 10 and are listed and elaborated on
in the following pages.
Shape - Shape may identify.
Size - Comparative sizes may identify.
Shadow - Shadow is often a clue by characteristic shape.
Tone/Color - Photographic tone or color is the characteristic colors
and/or gray shades of scenes and objects.
Texture - Texture in aerial photographs is created by the frequency of
tonal or color changes.
Pattern - Pattern is a more or less orderly arrangement of manmade objects
or natural elements.
Relation to surrounding objects - This subhead might just as well be
labeled "Deduction." Actually all interpretation hinges here to some degree,
consciously or unconsciously.
Shape . The shapes of objects seen in vertical view are sometimes surprisingly
difficult to interpret. The plan or top view of an object is so different
from the familiar profile or oblique view that inexperienced interpreters have
failed to recognize the image of the building in which they were working. The
ability to understand and make use of the plan view has to be acquired like
another language. It then becomes a powerful tool, for the plan view of
objects is an important and sometimes conclusive indication of their
structure, composition, and function. To the interpreter who is experienced
in industrial studies, the vertical view of a factory tells more about its
function than a stroll past its front door. The vertical view of a forest may
reveal its economic and recreational value. The vertical view of a landform
may show spectacular effects of tectronic and gradational processes. To the
motorist, a cloverleaf road intersection is an incomprehensible maze through
which one must find his/her way by faith and strict attention to signs; to the
aerial observer, the intersection is perfectly clear in form and function.
Much of the training of the photo interpreter is aimed at the
reorientation of perceptions; so that he/she can easily recognize objects seen
from above. This reorientation is greatly aided by the impression of depth in
Size . The size of an object is one of the most useful clues to its identity;
by measuring an unknown object on an aerial photograph, the interpreter can
eliminate from consideration whole groups of possible identifications. An
irrigation ditch and an anti-tank ditch, for example, are very much alike
except in size and a simple measurement may suffice to make the
identification. It is always advisable, when faced with an unknown object, to
measure it. When working with photography on variable scale, the interpreter
should make frequent measurements of the objects of interest.
The interpreter can avoid errors in identification by paying attention to
the size of objects. Misidentif ications are possible even though the
interpreter may have carefully considered such clues as shapes, shadow, tone,
texture, and pattern.
Shadows . Shadows are familiar phenomena, and in ordinary life we often judge
the size and shape of objects or persons by observing the shadows they cast.
The shadows present in aerial photographs sometimes help the interpreter by
providing profile representations of objects of interest. Shadows are
particularly helpful if the objects are very small or lack tonal contrast with
their surroundings. Under these conditions the sharp tonal gradients of the
shadows may enable the interpreter to identify objects which themselves are
just at the threshold of recognition.
b. Relative size and pattern
(A) SMOOTH SURFACE
(B) ROUGH SURFACE
(C) SLANTING SURFACE
(E) ROUGH TEXTURE
(C) (D) (E)
e. Relation to surroundings
CAST ON RIVER
Figure 9. Identification of objects.
Useful for large objects and large scale photography.
Shadows identify objects as a wind
break of Lombardy Poplar.
Shadow identifies object as a water
storage tank .
Watch out for cloud shadows !
Cloud shadows give a dark tone that
bears no relationship to the terrain.
However, in flat level country, these
shadows may puzzle photo users.
Sometimes there are shadows without distinguishable objects.
POWER POLES ARE
AT BASE OF SHADOW*
■ m. m .wf . jA"3> Vfi ^ CT=l
Figure 10. Shadows.
Tone . Tone of a photograph is a relative blackness or whiteness and is the
result of the amount of light reflected by an object. Tone is fundamental to
interpretation of black-and-white photography and, used with other recognition
elements, is a primary element for feature identification and interpretation.
The tones of photographic images are influenced by many factors, and the tones
of familiar objects often fail to correspond to our perceptions of those
objects in nature. A body of waste may appear in tones ranging from white to
black, depending on the angle of Sun and the number of wave surfaces
reflecting light to the camera lens. A black asphalt road may appear very
light in tone because of its smooth surface. A trail may appear white in dry
weather and dark after a rain. A smooth, rounded, metal object of
indeterminate color, like a tank truck, may reflect so much light that the
tonal range of the film cannot record it and details are lost. Figure 9
illustrates degrees of reflection, under "tone."
When the photo interpreter understands the factors which govern
photographic tone, he/she regards the tones of objects of interest as major
clues to their identity or composition. The soil scientist uses tonal
variations to classify soils; the forester, to distinguish hardwood from
coniferous trees; the geologist, to map lithology and structure or prospect
for minerals. In single photographs, where the shapes of objects must be
inferred from monocular clues, or in stereoscopic pairs if the objects of
interest have little or no visible height, tone is particularly important.
Color . A feature has a color when it reflects particular wavelengths of
light. For example, vegetation appears green because the properties of the
plant preferentially reflect a larger percent of green light compared to blue
and red. Hue (color), value (saturation), and chroma (brightness) are the
three variables that constitute color. The human eye can distinguish many
more hues of color than it can tones of gray. Thus color permits recognition
and interpretation of a greater amount of detail. In the interpretation of
rocks, soils, and plants where there may be an abundance of features whose
natural colors are important, some workers have found that color photography
pays for itself in abundance and accuracy of information. False-color
infrared film, developed during World War II, has been found useful for
special studies of plant conditions, vegetation distribution, soil-moisture
conditions and drainage delineation.
Texture . Relations of texture on tone are illustrated in Figure 11. Heavy
grazing removes many of the tiny traps of sunlight — grass blades. Reflection
is high with the grasses removed so the tones grow light. Ungrazed grasses
give darker tones. As grazing is seldom the same on both sides of the fence,
fences may be located by this difference in texture and tone. Sometimes stock
will travel the fence line, removing all vegetation and making the surface
very smooth. The result is a white line or trail paralleling the fence.
Taller grasses trap more light and reflect little toward the camera.
Meadows with greater densities of grass will be darker accordingly than
surrounding grassland. Shrubs and trees reflect little light and usually are
very dark. The tone will vary with individual species.
In general, the smoother the surface or texture, the lighter its tone in
the photograph. Dark tones come from rough textures — tall grasses and
shrubs. Color is often secondary in its effect on tone when it is combined
with texture. Check most carefully in puzzling areas where location is
important (as in pricking a section corner on a photograph) to assure that the
effects of texture on tone is evaluated properly.
Texture in aerial photographs is created by
color change in groups of objects which are too
individuals. It follows that the size of object
varies with the scale of photography. In large-
be seen as individuals; their leaves or needles
separately, but contribute to the texture of the
of smaller scale, the crowns contribute to the t
trees. Within a given range of scales, the text
(e.g., a timber stand of a certain species compo
enough to serve as a reliable clue to the identi
the frequency of tonal or
small to be discerned as
required to produce texture
scale photographs, trees can
cannot be discerned
tree crowns. In photographs
exture of the whole stand of
ure of a group of objects
sition) may be distinctive
ty of the objects.
Texture is an important interpretative factor in using images acquired
from orbital altitudes. For example, the relative erosional dissection of
area may be inferred only by its texture because individual drainageways
cannot be delineated. Drainage pattern is an important indicator of the type
of surficial materials and bedrock. In other cases, features may be similar
in color but may exhibit considerable difference in texture (e.g., volcanic
fields versus desert pavement).
Fence shows on photograph
as a deviding line between
the two gray tones . ->.
Rougher texture, tall grasses
with great density
Darker gray tone
Light gray tone
Dark gray tone
Coarse texture of willows
Figure II. Effect of surface and texture on black and white photographs
PATTERN MAY HELP TO IDENTIFY OBJECTS
Open level range with lighter meandering
tones converging could indicate a windmil
Trails and smooth textures from closer
grazing gives pattern.
Tanks and surrounding retention dikes
indicate oil storage.
PATTERN MAY INDICATE SOIL CONDITIONS
Very sandy soils or sands migrating over clay sub soil .
Figure 12. Patterns.
Patterns . Students of earth science have always laid great stress on the
pattern or spatial arrangement of objects as an important clue to their origin
or function. Geographers and anthropologists study settlement patterns and
their distribution in order to understand the effects of diffusion and
migration in cultural history. Outcrop patterns provide clues to geologic
structure, lithology, and soil texture. The varying relations between
organisms and their environment produce characteristic patterns of plant
Regional patterns which formerly could be studied only through laborious
ground observation are instantly and clearly visible in aerial and space
photographs. Aerial photographs capture many small but significant patterns
which might be overlooked or misinterpreted by the ground observer (e.g.,
fracture traces and "halos"). Innumerable variations in classic patterns can
be seen and exploited by means of image interpretation.
Some patterns are primarily cultural and others are primarily natural.
There are, however, few parts of the world which have not been affected by
man, and most of the patterns visible in imagery result from the interaction
of natural and cultural factors. Even the photogeologist must sometimes take
the activities of man into account.
Cultural features are conspicuous in aerial photographs because they
consist of straight lines or other regular configurations. Most of man's
activities leave scars on the earth which persist for a long time even after
the activities have ceased. Patterns of settlement, mining, and agriculture
may be visible from the air after thousands of years, directly or through
altered patterns of vegetation and erosion, and photo interpretation has
become an important technique of archaeology.
Patterns formed by agricultural practices, fracture alignments, drainage
networks, and vegetation are important factors in the interpretation of an
image. As mentioned above, intricate patterns may be reflected as a textural
difference in space imagery. In many instances, regional patterns associated
with other image elements provide clues to finer-grained patterns.
Patterns indicate relationships through repetition or arrangement of
objects or form. The relationship may be the typical patterns imposed on a
streambeo by geology, for example:
Trellis (tilted or
developed joints and
The relationship may be typical patterns of land use, the symmetry of oil
tanks surrounded by circling dikes, and soil conditions (Figure 12).
Parallel ridges might be a geological structure, but road net leading to
railway siding indicates strip mining.
Diversion dam with ditch might be
irrigation canal, but ditch is leading
to tailings which indicates a flume
used in a mining operation .
Figure 13. Relationship to surrounding objects.
Relationship . A link must be established in the chain of identification on a
photograph. This link is the summation of the relationships of objects on the
photograph. The other guides or keys to identification and interpretation
sometimes are not enough.
A railroad may be easily identified (Figure 13). It ends in a field
abruptly. In time of war, it could well be a railroad gun or a guided missile
site, especially with a spur parallel to it. However, a sugarcane field might
be ready for harvesting and these are the temporary tracks. The
identification requires a relationship not only to the ground but to many
factors implicit to the situation on the ground but perhaps incapable of being
expressed in a photograph. This is where you, your background, and experience
lead to a correct interpretation of the photograph.
A broad knowledge of cultural, economic and social factors means much in
this phase of interpretation. A single example is given in Figure 9. A road
going to a stream and continuing on the opposite bank, plus the lack of a
bridge, is an indication of a ford or a ferry. The parking lot for cars
awaiting for the return trip suggest a ferry.
Knowledge of local customs that influence structural design may help.
Churches in Europe are oriented toward the east. The direction of the church
is determined by the cross-like character of the building — east is at the
altar. Customary driving on the roads (left or right) affects construction of
cloverleaf intersections — an indication of the unconscious impact of customs
of a country on design.
The obvious relationship of railroads to sidings, the abrupt curves of
secondary roads, the broad sweep of a super-highway are more than just that.
They represent our evaluation of the everyday happenings in our like as we see
them, readily recognize the conditions, and relate them to the photograph. If
we carry this relationship further in careful evaluation of conditions, not
only on manmade features but geological and soil features as well, as a cause
and effect, we are entering into the highly skilled field of photo
interpretation where many of the Bureau employees rightfully belong.
Techniques and Procedures
Certain image-interpretation techniques, when properly applied, can
improve the quality and quantity of useful information extracted from
imagery. Among the techniques being refined are methods for using (1)
methodical procedures, (2) efficient search techniques, (3) knowledge of
factors-governing image formation, (4) the background and training of the
interpreter, (5) the concept of "convergence of evidence," (6) the "conference
system," (7) information available in analogous areas, (8) reference
materials, (9) simple and sophisticated equipment, and (10) acquiring field
The image interpreter must have an understanding of: (1) how the image
was formed, (2) what the image elements represent, and (3) the basic earth
processes and phenomena that are present on the image. The amount of
information that an interpreter can extract from an image is proportional to
his/her experience, skill, and interest.
Efficiency in the analysis of imagery is dependent upon a number of
factors. A systematic approach to the problem under study is perhaps the most
important consideration. In general, a systematic analysis involves the
convergence of empirical evidence drawn from the imagery. The following
procedures (steps) illustrate this method:
3. Collateral information
c. Field data
4. Summary of interpretation results
5. Field check in key areas
1. Regional analysis
a. Geographic aspects
b. Physiographic aspects
2. Local analysis
a. Land forms
b. Drainage patterns
c. Erosion .
d. Image patterns
f. Spectral features
The approach works and results in a savings of time. Additional benefits
and information are also obtained by using a multidisciplinary/
interdisciplinary team approach.
The Effective Area is that central portion of a vertical photograph delimited
by the bisectors of the overlaps with adjacent photographs. On a vertical
photograph, all images within the effective area have less displacement than
their conjugate images on adjacent photographs.
Effective area boundaries are important in photo interpretation for two
reasons: (1) on photography of mountainous country, they define the smallest
usable area of least displacement on each photograph, and (2) they provide
interpretation "match lines" between photo pairs which avoid duplication or
gaps in delineation between photographs — both within, and between, lines.
When determining effective area boundaries proceed as explained and
illustrated in the following, (Substitute your own numbers instead of the
In-Line Effective Area Boundaries .
o Place photo 143 over photo 144 in overlap position with flight line segments
coinciding (Step 1, Figure 14). Don't tape down.
o Visually locate and pencil mark, the endlap midpoint on the west side of
photo 144. Now slide 143 across 144 perpendicular to the line of flight until
the format margin on the east side of 144 is showing. Mark, the endlap
midpoint on the east side of 144 (Steps 1 and 2, Figure 14).
o Connect these overlap midpoints on 144 with a ruler and colored lead pencil.
o Stereoscopical ly , or visually without a stereoscope, transfer 4-5 points
along this mid-overlap line to photo 143. Do not pinprick! Accomplish
quickly with pencil and locate the points at extremes of relief (ridge tops,
valley bottoms, etc.).
o Connect the transferred pencil points on 143 with colored pencil (Figure
15). If there is considerable variation in relief, the transferred boundary
will be crooked — and it is! Viewed stereoscopical ly , if properly
accomplished, the two lines will merge and "flow" over the surface of the
o Locate the in-line effective area boundaries between stereo pairs 143-142,
229-230, and 230-231.
o On the end photos in the flight lines (142, 144, 229, 231), rule straight
pencil lines through the Principal Points (PPs) (Photo Centers), across the
line of flight, to within about 1" of the east and west fiducial marks on each
of these photos as in Step 1, Figure 16.
Between-Line Effective Area Boundaries .
o Place the two lines of photography side-by-side in front of you with the
titling away from you (Step 1, Figure 16).
o Overlap 229 over 144 on the basis of matching ground features to determine
the sidelap zone.
v — Pencil mai
— __ _
- — Slide 144 in this
direction to expose
edge of photo 143
Location of endlap
Figure 15. Transfer of endlap bisector
on photo 144 to 143 results in
a crooked line on 143 due to
■ — c
In the absence of an adjoining line of
photos, this boundary is drawn about
1.25 inches from the photo edge.
Figure 16. Location of the in-line
effective area boundaries.
o Visually locate the sidelap midpoints and rule in as a straight line on
229. Do not extend the line beyond the endlap effective area boundaries (Step
2, Figure 16).
o Visually transfer this line to its conjugate position on photo 144. When
there are extremes in relief in the sidelap zone, this line will be irregular
— reflecting variations in topographic displacement (Step 3, Figure 16).
o Repeat procedure between 143 and 230, 142, and 231.
o Since there are no adjoining photographs on the east of 229-231, or on the
west of 142-144, put in the final side of each effective area at about 15% of
the width of the photo from the edge (circa 1.25 inches) - as illustrated in
Step 4, Figure 16, for photos 143 and 230.
o Please note that, where transferred boundaries are indicated in the above
diagrams, the degree of irregularity has been purposely exaggerated.
o It is not unusual for the effective area boundaries to sometimes be located
on alternate photos - particularly in areas with little, or no, steep
topography. In such cases, the effective area boundaries are determined in
precisely the manner described above - but between every-other photo.
Overlay Preparation :
Clear acetate overlays (9 in. x 9 in.) should be placed on each frame to be
interpreted. A fine point, permanent marking pen should be used. Delineate a
line over each fiducial mark observed on the photo. Trace the photo number
(e.g., 143) precisely over the lettering on the print. Trace the match lines
precisely from the print. The interpretation overlay is now registered to
either a particular transparency or its paper print. It can be removed from
over the photo and replaced without fear of misregistration. Figure 17 shows
examples of properly prepared overlays.
Interpret this area
between match lines
for each photo .
Figure 18 is an overlay representing typical classifications of vegetation
within a specific area. Symbols O/P is oak and pine with oak being dominant
and pine sub-dominant. D/P is Douglas fir and pine, O/G is oak and grass, etc.
Figure 19 is a typical drainage overlay.
APPROXIMATE SCALE = 1:18,600
Figure 18. Typical vegetation overlay.
APPROXIMATE SCALE = I : 18,600
Figure 19. Typical drainage overlay.
I wish to express my sincere appreciation to the following individuals for the
use of information originally prepared by them for special purposes and/or
presentat ions :
Donald T. Lauer (Chief, Technique Development and Applications Branch, EROS
Data Center, U.S. Geological Survey), Dr. Merle Meyer (University of
Minnesota-Retired), and Richard Burr (BLM-Retired) . Special recognition is
appropriate for Herman E. Weiss (BLM-Visual Information Specialist) for his
invaluable assistance with the development of illustrations and with
requirements for printing this publication.
1. REPORT NO.
4. Title and Subtitle
Interpretation of Aerial Photographs
Wallace A. Crisco
9. Performing Organization Name and Address
U.S. Department of the Interior
Bureau of Land Management Service Center
P.O. Box 25047
Denver, CO 80225-0047
3. Recipient's Accession No.
5. Report Date
8. Performing Organization Rept. No.
10. Project/Task/Work Unit No.
11. Contract(C) or Grant(G) No.
12. Sponsoring Organization Name and Address
U.S. Department of the Interior
Bureau of Land Management Service Center
P.O. Box 25047
Denver. CO 80225-0047
13. Type of Report & Period Covered
15. Supplementary Notes
16. Abstract (Limit: 200 words)
This publication provides information on basic interpretation of aerial photographs,
Included are discussions on types of photography, films, and filters. Photo
interpretation and usage are also covered, with discussions of scale and acreage
determinations, image characteristics, and photo preparation.
17. Document Analysis a. Descriptors
b. Identifiers/Open-Ended Terms
c. COSATI field/Group
18. Availability Statement
19. Security Class (This Report)
20. Security Class (This Page)
21. No. of Pages
See Instructions on Reverse
OPTIONAL FORM 272 (4-77)
Department of Commerce
ii U.S. GOVERNMENT PRINTING OFFICE: 1988—573-003/80,819 REGION NO 8
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P.O. Bos 25047 i
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